Fluids, for example paints, are applied by means of high-speed bell atomizers which, by virtue of their shape, act on the inside like a turbo machine, i.e. they suck gas in from the surrounding atmosphere, deflect this gas and transport it back to the outside, so that a toroidal vortex forms inside and in front of the bell cavity.
In the application of fluids which evaporate quickly, for example, metallic base paints which are currently widely used, the gas contains very fine particles, for example paint particles, which deposit on the hub plate of the bell atomizer and contaminate this hub plate. Since the deposited particles no longer flow freely, if at all, solids-rich ridges grow on the hub plate until after a short period--which depends on their size--they are flung from the hub plate and pass into the paint film guided by the bell atomizer and from there onto the article to be coated or painted, where they cause considerable surface imperfections. This process of contaminating the hub plate and the tearing-off of these deposited ridges takes place very rapidly, so that a painting process cannot usually be completed without the appearance of the above-mentioned surface imperfections.
In the case of fluids, for example paints, which evaporate slowly, these particles are still sufficiently moist when arriving on the hub plate for them not to adhere, and the paint particles deposited on the hub plate are immediately radially accelerated on the hub plate and pass into the freshly metered paint stream without accumulating and hence being the cause of surface imperfections on the article being painted.
In the case of high-viscosity or low-viscosity paints or fluids, the devices described in German Pat. No. 3,005,677 and German Offenlegungsschrift No. 3,047,670 respectively provide an adequate solution. However, in the case of paints which evaporate quickly, for example, metallic base paints, these known devices provide no guidance for avoiding the difficulties described above.
It has also become known in practice to wet the hub plate, via a by-pass flow, with fresh paint. This way of solving the problem tends to lead to contamination in the narrow bore, since the bore has to have such a small diameter as to set the by-pass flow in rotation as it passes through. To ensure that the by-pass functions, these systems need to be fed with the material in the center, and that means it is not possible to mount separate feed ducts for incompatible fluids and for solvents, so that the possibilities of rapid color change are strictly limited in these existing devices. The existing type of device is also frequently prone to color entrainment effects. In addition, inadequate rinsing can lead to blockages in the bore and hence to wetting problems on the hub plate.